Heating oil fumes that are detectable by smell are not dangerous. However, odorless carbon monoxide fumes from a heater are dangerous. Detectable fumes do indicate a problem within the heating system that usually requires repair.
Heating oil fumes can be caused by a burner ignition problem that causes smoke from unburned heating oil to circulate through heating ducts and into the home. While these smoke fumes are harmless, their presence signals there is a crack or misalignment of some type within the oil burner. Soot from the smoke might also adhere to walls near heating vents. A heating system service technician can usually fix the burner problem.
Carbon monoxide is produced as a byproduct of a combustible heating system, and is always present when fuel is burned. Normally, carbon monoxide is released into the atmosphere safely via the home's ventilation system. However, in some instances, carbon monoxide builds up to dangerous levels due to a crack in the heat exchanger. The combination of blocked vents and a malfunctioning heat exchanger can lead to deadly carbon monoxide fumes filling the home. The only way to detect carbon monoxide fumes in the home is to install a safety-approved carbon monoxide detector, which operates in a way similar to a smoke detector.